Im curious how flight operations work with Fedex sweep flights. An example lets say SEA to MEM. I imagine a route would be fly north over Boise, Calgary, Billings, Minneapolis before turning south to Memphis. If a flight needed to divert to pick up extra cargo, how does this work? I know they would inform ATC about diverting to a city. But how about IFR alternates. And calculating airplane performance at that airport.

I don't work there, but it should be pretty simple- verify you'll be at an appropriate landing weight, the weather is legal for redispatch, and drop in with the usual coordination as a usual divert.

I should add that at my airline we have no requirement to check landing performance (as long as you are below max landing weight) at online runways above 7000 feet in length on non-contaminated runways with no performance inhibiting mx deferrals. We are guaranteed performance if meeting those conditions.

I also do not work at FX any more, But i do work for the other team now. We don't usually have designated "sweep" flights but we do often divert flights to recover volume for whatever reason. The operations have become fairly routine to get the "hey, we need flight 123 in OMA". From the dispatcher point of view, you check the weather, make sure you have a safe/legal amount of fuel to make it there. If the new dest requires an alternate, you add that fuel requirement in.

As far as runway perf, we can check it easily enough. The crews can use operation manuals to get min field length requirements and climb limits. Domestically there are very few restrictive runways we operate to and structural weight becomes the limit.

It's quite commonplace for us that the crews/dispatchers pretty much have the routine down.

I did work there and and the key facts are you never dept early and you flight the route as filed, no shortcuts. There's not too much of a chance ldg wgt will be a problem and if they need you to drop into a city it's obviously closer than going all the way to MEM so fuel isn't a problem either. I did do a SEA-MEM sweep but it went south over PHX where we did drop into. New flightplan, more fuel, freight and away you go. not a big deal really.